/*
 * Copyright (c) 1994, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.lang;

import java.io.*;
import java.lang.reflect.Executable;
import java.lang.annotation.Annotation;
import java.security.AccessControlContext;
import java.util.Properties;
import java.util.PropertyPermission;
import java.util.StringTokenizer;
import java.util.Map;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.AllPermission;
import java.nio.channels.Channel;
import java.nio.channels.spi.SelectorProvider;
import sun.nio.ch.Interruptible;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
import sun.security.util.SecurityConstants;
import sun.reflect.annotation.AnnotationType;

/**
 * The <code>System</code> class contains several useful class fields
 * and methods. It cannot be instantiated.
 *
 * <p>Among the facilities provided by the <code>System</code> class
 * are standard input, standard output, and error output streams;
 * access to externally defined properties and environment
 * variables; a means of loading files and libraries; and a utility
 * method for quickly copying a portion of an array.
 *
 * @author unascribed
 * @since JDK1.0
 */
public final class System {

  /* register the natives via the static initializer.
   *
   * VM will invoke the initializeSystemClass method to complete
   * the initialization for this class separated from clinit.
   * Note that to use properties set by the VM, see the constraints
   * described in the initializeSystemClass method.
   */
  private static native void registerNatives();

  static {
    registerNatives();
  }

  /**
   * Don't let anyone instantiate this class
   */
  private System() {
  }

  /**
   * The "standard" input stream. This stream is already
   * open and ready to supply input data. Typically this stream
   * corresponds to keyboard input or another input source specified by
   * the host environment or user.
   */
  public final static InputStream in = null;

  /**
   * The "standard" output stream. This stream is already
   * open and ready to accept output data. Typically this stream
   * corresponds to display output or another output destination
   * specified by the host environment or user.
   * <p>
   * For simple stand-alone Java applications, a typical way to write
   * a line of output data is:
   * <blockquote><pre>
   *     System.out.println(data)
   * </pre></blockquote>
   * <p>
   * See the <code>println</code> methods in class <code>PrintStream</code>.
   *
   * @see java.io.PrintStream#println()
   * @see java.io.PrintStream#println(boolean)
   * @see java.io.PrintStream#println(char)
   * @see java.io.PrintStream#println(char[])
   * @see java.io.PrintStream#println(double)
   * @see java.io.PrintStream#println(float)
   * @see java.io.PrintStream#println(int)
   * @see java.io.PrintStream#println(long)
   * @see java.io.PrintStream#println(java.lang.Object)
   * @see java.io.PrintStream#println(java.lang.String)
   */
  public final static PrintStream out = null;

  /**
   * The "standard" error output stream. This stream is already
   * open and ready to accept output data.
   * <p>
   * Typically this stream corresponds to display output or another
   * output destination specified by the host environment or user. By
   * convention, this output stream is used to display error messages
   * or other information that should come to the immediate attention
   * of a user even if the principal output stream, the value of the
   * variable <code>out</code>, has been redirected to a file or other
   * destination that is typically not continuously monitored.
   */
  public final static PrintStream err = null;

  /* The security manager for the system.
   */
  private static volatile SecurityManager security = null;

  /**
   * Reassigns the "standard" input stream.
   *
   * <p>First, if there is a security manager, its <code>checkPermission</code>
   * method is called with a <code>RuntimePermission("setIO")</code> permission
   * to see if it's ok to reassign the "standard" input stream.
   * <p>
   *
   * @param in the new standard input stream.
   * @throws SecurityException if a security manager exists and its <code>checkPermission</code>
   * method doesn't allow reassigning of the standard input stream.
   * @see SecurityManager#checkPermission
   * @see java.lang.RuntimePermission
   * @since JDK1.1
   */
  public static void setIn(InputStream in) {
    checkIO();
    setIn0(in);
  }

  /**
   * Reassigns the "standard" output stream.
   *
   * <p>First, if there is a security manager, its <code>checkPermission</code>
   * method is called with a <code>RuntimePermission("setIO")</code> permission
   * to see if it's ok to reassign the "standard" output stream.
   *
   * @param out the new standard output stream
   * @throws SecurityException if a security manager exists and its <code>checkPermission</code>
   * method doesn't allow reassigning of the standard output stream.
   * @see SecurityManager#checkPermission
   * @see java.lang.RuntimePermission
   * @since JDK1.1
   */
  public static void setOut(PrintStream out) {
    checkIO();
    setOut0(out);
  }

  /**
   * Reassigns the "standard" error output stream.
   *
   * <p>First, if there is a security manager, its <code>checkPermission</code>
   * method is called with a <code>RuntimePermission("setIO")</code> permission
   * to see if it's ok to reassign the "standard" error output stream.
   *
   * @param err the new standard error output stream.
   * @throws SecurityException if a security manager exists and its <code>checkPermission</code>
   * method doesn't allow reassigning of the standard error output stream.
   * @see SecurityManager#checkPermission
   * @see java.lang.RuntimePermission
   * @since JDK1.1
   */
  public static void setErr(PrintStream err) {
    checkIO();
    setErr0(err);
  }

  private static volatile Console cons = null;

  /**
   * Returns the unique {@link java.io.Console Console} object associated
   * with the current Java virtual machine, if any.
   *
   * @return The system console, if any, otherwise <tt>null</tt>.
   * @since 1.6
   */
  public static Console console() {
    if (cons == null) {
      synchronized (System.class) {
        cons = sun.misc.SharedSecrets.getJavaIOAccess().console();
      }
    }
    return cons;
  }

  /**
   * Returns the channel inherited from the entity that created this
   * Java virtual machine.
   *
   * <p> This method returns the channel obtained by invoking the
   * {@link java.nio.channels.spi.SelectorProvider#inheritedChannel
   * inheritedChannel} method of the system-wide default
   * {@link java.nio.channels.spi.SelectorProvider} object. </p>
   *
   * <p> In addition to the network-oriented channels described in
   * {@link java.nio.channels.spi.SelectorProvider#inheritedChannel
   * inheritedChannel}, this method may return other kinds of
   * channels in the future.
   *
   * @return The inherited channel, if any, otherwise <tt>null</tt>.
   * @throws IOException If an I/O error occurs
   * @throws SecurityException If a security manager is present and it does not permit access to the
   * channel.
   * @since 1.5
   */
  public static Channel inheritedChannel() throws IOException {
    return SelectorProvider.provider().inheritedChannel();
  }

  private static void checkIO() {
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      sm.checkPermission(new RuntimePermission("setIO"));
    }
  }

  private static native void setIn0(InputStream in);

  private static native void setOut0(PrintStream out);

  private static native void setErr0(PrintStream err);

  /**
   * Sets the System security.
   *
   * <p> If there is a security manager already installed, this method first
   * calls the security manager's <code>checkPermission</code> method
   * with a <code>RuntimePermission("setSecurityManager")</code>
   * permission to ensure it's ok to replace the existing
   * security manager.
   * This may result in throwing a <code>SecurityException</code>.
   *
   * <p> Otherwise, the argument is established as the current
   * security manager. If the argument is <code>null</code> and no
   * security manager has been established, then no action is taken and
   * the method simply returns.
   *
   * @param s the security manager.
   * @throws SecurityException if the security manager has already been set and its
   * <code>checkPermission</code> method doesn't allow it to be replaced.
   * @see #getSecurityManager
   * @see SecurityManager#checkPermission
   * @see java.lang.RuntimePermission
   */
  public static void setSecurityManager(final SecurityManager s) {
    try {
      s.checkPackageAccess("java.lang");
    } catch (Exception e) {
      // no-op
    }
    setSecurityManager0(s);
  }

  private static synchronized void setSecurityManager0(final SecurityManager s) {
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      // ask the currently installed security manager if we
      // can replace it.
      sm.checkPermission(new RuntimePermission
          ("setSecurityManager"));
    }

    if ((s != null) && (s.getClass().getClassLoader() != null)) {
      // New security manager class is not on bootstrap classpath.
      // Cause policy to get initialized before we install the new
      // security manager, in order to prevent infinite loops when
      // trying to initialize the policy (which usually involves
      // accessing some security and/or system properties, which in turn
      // calls the installed security manager's checkPermission method
      // which will loop infinitely if there is a non-system class
      // (in this case: the new security manager class) on the stack).
      AccessController.doPrivileged(new PrivilegedAction<Object>() {
        public Object run() {
          s.getClass().getProtectionDomain().implies
              (SecurityConstants.ALL_PERMISSION);
          return null;
        }
      });
    }

    security = s;
  }

  /**
   * Gets the system security interface.
   *
   * @return if a security manager has already been established for the current application, then
   * that security manager is returned; otherwise, <code>null</code> is returned.
   * @see #setSecurityManager
   */
  public static SecurityManager getSecurityManager() {
    return security;
  }

  /**
   * Returns the current time in milliseconds.  Note that
   * while the unit of time of the return value is a millisecond,
   * the granularity of the value depends on the underlying
   * operating system and may be larger.  For example, many
   * operating systems measure time in units of tens of
   * milliseconds.
   *
   * <p> See the description of the class <code>Date</code> for
   * a discussion of slight discrepancies that may arise between
   * "computer time" and coordinated universal time (UTC).
   *
   * @return the difference, measured in milliseconds, between the current time and midnight,
   * January 1, 1970 UTC.
   * @see java.util.Date
   */
  public static native long currentTimeMillis();

  /**
   * Returns the current value of the running Java Virtual Machine's
   * high-resolution time source, in nanoseconds.
   *
   * <p>This method can only be used to measure elapsed time and is
   * not related to any other notion of system or wall-clock time.
   * The value returned represents nanoseconds since some fixed but
   * arbitrary <i>origin</i> time (perhaps in the future, so values
   * may be negative).  The same origin is used by all invocations of
   * this method in an instance of a Java virtual machine; other
   * virtual machine instances are likely to use a different origin.
   *
   * <p>This method provides nanosecond precision, but not necessarily
   * nanosecond resolution (that is, how frequently the value changes)
   * - no guarantees are made except that the resolution is at least as
   * good as that of {@link #currentTimeMillis()}.
   *
   * <p>Differences in successive calls that span greater than
   * approximately 292 years (2<sup>63</sup> nanoseconds) will not
   * correctly compute elapsed time due to numerical overflow.
   *
   * <p>The values returned by this method become meaningful only when
   * the difference between two such values, obtained within the same
   * instance of a Java virtual machine, is computed.
   *
   * <p> For example, to measure how long some code takes to execute:
   * <pre> {@code
   * long startTime = System.nanoTime();
   * // ... the code being measured ...
   * long estimatedTime = System.nanoTime() - startTime;}</pre>
   *
   * <p>To compare two nanoTime values
   * <pre> {@code
   * long t0 = System.nanoTime();
   * ...
   * long t1 = System.nanoTime();}</pre>
   *
   * one should use {@code t1 - t0 < 0}, not {@code t1 < t0},
   * because of the possibility of numerical overflow.
   *
   * @return the current value of the running Java Virtual Machine's high-resolution time source, in
   * nanoseconds
   * @since 1.5
   */
  public static native long nanoTime();

  /**
   * Copies an array from the specified source array, beginning at the
   * specified position, to the specified position of the destination array.
   * A subsequence of array components are copied from the source
   * array referenced by <code>src</code> to the destination array
   * referenced by <code>dest</code>. The number of components copied is
   * equal to the <code>length</code> argument. The components at
   * positions <code>srcPos</code> through
   * <code>srcPos+length-1</code> in the source array are copied into
   * positions <code>destPos</code> through
   * <code>destPos+length-1</code>, respectively, of the destination
   * array.
   * <p>
   * If the <code>src</code> and <code>dest</code> arguments refer to the
   * same array object, then the copying is performed as if the
   * components at positions <code>srcPos</code> through
   * <code>srcPos+length-1</code> were first copied to a temporary
   * array with <code>length</code> components and then the contents of
   * the temporary array were copied into positions
   * <code>destPos</code> through <code>destPos+length-1</code> of the
   * destination array.
   * <p>
   * If <code>dest</code> is <code>null</code>, then a
   * <code>NullPointerException</code> is thrown.
   * <p>
   * If <code>src</code> is <code>null</code>, then a
   * <code>NullPointerException</code> is thrown and the destination
   * array is not modified.
   * <p>
   * Otherwise, if any of the following is true, an
   * <code>ArrayStoreException</code> is thrown and the destination is
   * not modified:
   * <ul>
   * <li>The <code>src</code> argument refers to an object that is not an
   * array.
   * <li>The <code>dest</code> argument refers to an object that is not an
   * array.
   * <li>The <code>src</code> argument and <code>dest</code> argument refer
   * to arrays whose component types are different primitive types.
   * <li>The <code>src</code> argument refers to an array with a primitive
   * component type and the <code>dest</code> argument refers to an array
   * with a reference component type.
   * <li>The <code>src</code> argument refers to an array with a reference
   * component type and the <code>dest</code> argument refers to an array
   * with a primitive component type.
   * </ul>
   * <p>
   * Otherwise, if any of the following is true, an
   * <code>IndexOutOfBoundsException</code> is
   * thrown and the destination is not modified:
   * <ul>
   * <li>The <code>srcPos</code> argument is negative.
   * <li>The <code>destPos</code> argument is negative.
   * <li>The <code>length</code> argument is negative.
   * <li><code>srcPos+length</code> is greater than
   * <code>src.length</code>, the length of the source array.
   * <li><code>destPos+length</code> is greater than
   * <code>dest.length</code>, the length of the destination array.
   * </ul>
   * <p>
   * Otherwise, if any actual component of the source array from
   * position <code>srcPos</code> through
   * <code>srcPos+length-1</code> cannot be converted to the component
   * type of the destination array by assignment conversion, an
   * <code>ArrayStoreException</code> is thrown. In this case, let
   * <b><i>k</i></b> be the smallest nonnegative integer less than
   * length such that <code>src[srcPos+</code><i>k</i><code>]</code>
   * cannot be converted to the component type of the destination
   * array; when the exception is thrown, source array components from
   * positions <code>srcPos</code> through
   * <code>srcPos+</code><i>k</i><code>-1</code>
   * will already have been copied to destination array positions
   * <code>destPos</code> through
   * <code>destPos+</code><i>k</I><code>-1</code> and no other
   * positions of the destination array will have been modified.
   * (Because of the restrictions already itemized, this
   * paragraph effectively applies only to the situation where both
   * arrays have component types that are reference types.)
   *
   * @param src the source array.
   * @param srcPos starting position in the source array.
   * @param dest the destination array.
   * @param destPos starting position in the destination data.
   * @param length the number of array elements to be copied.
   * @throws IndexOutOfBoundsException if copying would cause access of data outside array bounds.
   * @throws ArrayStoreException if an element in the <code>src</code> array could not be stored
   * into the <code>dest</code> array because of a type mismatch.
   * @throws NullPointerException if either <code>src</code> or <code>dest</code> is
   * <code>null</code>.
   */
  public static native void arraycopy(Object src, int srcPos,
      Object dest, int destPos,
      int length);

  /**
   * Returns the same hash code for the given object as
   * would be returned by the default method hashCode(),
   * whether or not the given object's class overrides
   * hashCode().
   * The hash code for the null reference is zero.
   *
   * @param x object for which the hashCode is to be calculated
   * @return the hashCode
   * @since JDK1.1
   */
  public static native int identityHashCode(Object x);

  /**
   * System properties. The following properties are guaranteed to be defined:
   * <dl>
   * <dt>java.version         <dd>Java version number
   * <dt>java.vendor          <dd>Java vendor specific string
   * <dt>java.vendor.url      <dd>Java vendor URL
   * <dt>java.home            <dd>Java installation directory
   * <dt>java.class.version   <dd>Java class version number
   * <dt>java.class.path      <dd>Java classpath
   * <dt>os.name              <dd>Operating System Name
   * <dt>os.arch              <dd>Operating System Architecture
   * <dt>os.version           <dd>Operating System Version
   * <dt>file.separator       <dd>File separator ("/" on Unix)
   * <dt>path.separator       <dd>Path separator (":" on Unix)
   * <dt>line.separator       <dd>Line separator ("\n" on Unix)
   * <dt>user.name            <dd>User account name
   * <dt>user.home            <dd>User home directory
   * <dt>user.dir             <dd>User's current working directory
   * </dl>
   */

  private static Properties props;

  private static native Properties initProperties(Properties props);

  /**
   * Determines the current system properties.
   * <p>
   * First, if there is a security manager, its
   * <code>checkPropertiesAccess</code> method is called with no
   * arguments. This may result in a security exception.
   * <p>
   * The current set of system properties for use by the
   * {@link #getProperty(String)} method is returned as a
   * <code>Properties</code> object. If there is no current set of
   * system properties, a set of system properties is first created and
   * initialized. This set of system properties always includes values
   * for the following keys:
   * <table summary="Shows property keys and associated values">
   * <tr><th>Key</th>
   * <th>Description of Associated Value</th></tr>
   * <tr><td><code>java.version</code></td>
   * <td>Java Runtime Environment version</td></tr>
   * <tr><td><code>java.vendor</code></td>
   * <td>Java Runtime Environment vendor</td></tr>
   * <tr><td><code>java.vendor.url</code></td>
   * <td>Java vendor URL</td></tr>
   * <tr><td><code>java.home</code></td>
   * <td>Java installation directory</td></tr>
   * <tr><td><code>java.vm.specification.version</code></td>
   * <td>Java Virtual Machine specification version</td></tr>
   * <tr><td><code>java.vm.specification.vendor</code></td>
   * <td>Java Virtual Machine specification vendor</td></tr>
   * <tr><td><code>java.vm.specification.name</code></td>
   * <td>Java Virtual Machine specification name</td></tr>
   * <tr><td><code>java.vm.version</code></td>
   * <td>Java Virtual Machine implementation version</td></tr>
   * <tr><td><code>java.vm.vendor</code></td>
   * <td>Java Virtual Machine implementation vendor</td></tr>
   * <tr><td><code>java.vm.name</code></td>
   * <td>Java Virtual Machine implementation name</td></tr>
   * <tr><td><code>java.specification.version</code></td>
   * <td>Java Runtime Environment specification  version</td></tr>
   * <tr><td><code>java.specification.vendor</code></td>
   * <td>Java Runtime Environment specification  vendor</td></tr>
   * <tr><td><code>java.specification.name</code></td>
   * <td>Java Runtime Environment specification  name</td></tr>
   * <tr><td><code>java.class.version</code></td>
   * <td>Java class format version number</td></tr>
   * <tr><td><code>java.class.path</code></td>
   * <td>Java class path</td></tr>
   * <tr><td><code>java.library.path</code></td>
   * <td>List of paths to search when loading libraries</td></tr>
   * <tr><td><code>java.io.tmpdir</code></td>
   * <td>Default temp file path</td></tr>
   * <tr><td><code>java.compiler</code></td>
   * <td>Name of JIT compiler to use</td></tr>
   * <tr><td><code>java.ext.dirs</code></td>
   * <td>Path of extension directory or directories
   * <b>Deprecated.</b> <i>This property, and the mechanism
   * which implements it, may be removed in a future
   * release.</i> </td></tr>
   * <tr><td><code>os.name</code></td>
   * <td>Operating system name</td></tr>
   * <tr><td><code>os.arch</code></td>
   * <td>Operating system architecture</td></tr>
   * <tr><td><code>os.version</code></td>
   * <td>Operating system version</td></tr>
   * <tr><td><code>file.separator</code></td>
   * <td>File separator ("/" on UNIX)</td></tr>
   * <tr><td><code>path.separator</code></td>
   * <td>Path separator (":" on UNIX)</td></tr>
   * <tr><td><code>line.separator</code></td>
   * <td>Line separator ("\n" on UNIX)</td></tr>
   * <tr><td><code>user.name</code></td>
   * <td>User's account name</td></tr>
   * <tr><td><code>user.home</code></td>
   * <td>User's home directory</td></tr>
   * <tr><td><code>user.dir</code></td>
   * <td>User's current working directory</td></tr>
   * </table>
   * <p>
   * Multiple paths in a system property value are separated by the path
   * separator character of the platform.
   * <p>
   * Note that even if the security manager does not permit the
   * <code>getProperties</code> operation, it may choose to permit the
   * {@link #getProperty(String)} operation.
   *
   * @return the system properties
   * @throws SecurityException if a security manager exists and its <code>checkPropertiesAccess</code>
   * method doesn't allow access to the system properties.
   * @see #setProperties
   * @see java.lang.SecurityException
   * @see java.lang.SecurityManager#checkPropertiesAccess()
   * @see java.util.Properties
   */
  public static Properties getProperties() {
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      sm.checkPropertiesAccess();
    }

    return props;
  }

  /**
   * Returns the system-dependent line separator string.  It always
   * returns the same value - the initial value of the {@linkplain
   * #getProperty(String) system property} {@code line.separator}.
   *
   * <p>On UNIX systems, it returns {@code "\n"}; on Microsoft
   * Windows systems it returns {@code "\r\n"}.
   *
   * @return the system-dependent line separator string
   * @since 1.7
   */
  public static String lineSeparator() {
    return lineSeparator;
  }

  private static String lineSeparator;

  /**
   * Sets the system properties to the <code>Properties</code>
   * argument.
   * <p>
   * First, if there is a security manager, its
   * <code>checkPropertiesAccess</code> method is called with no
   * arguments. This may result in a security exception.
   * <p>
   * The argument becomes the current set of system properties for use
   * by the {@link #getProperty(String)} method. If the argument is
   * <code>null</code>, then the current set of system properties is
   * forgotten.
   *
   * @param props the new system properties.
   * @throws SecurityException if a security manager exists and its <code>checkPropertiesAccess</code>
   * method doesn't allow access to the system properties.
   * @see #getProperties
   * @see java.util.Properties
   * @see java.lang.SecurityException
   * @see java.lang.SecurityManager#checkPropertiesAccess()
   */
  public static void setProperties(Properties props) {
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      sm.checkPropertiesAccess();
    }
    if (props == null) {
      props = new Properties();
      initProperties(props);
    }
    System.props = props;
  }

  /**
   * Gets the system property indicated by the specified key.
   * <p>
   * First, if there is a security manager, its
   * <code>checkPropertyAccess</code> method is called with the key as
   * its argument. This may result in a SecurityException.
   * <p>
   * If there is no current set of system properties, a set of system
   * properties is first created and initialized in the same manner as
   * for the <code>getProperties</code> method.
   *
   * @param key the name of the system property.
   * @return the string value of the system property, or <code>null</code> if there is no property
   * with that key.
   * @throws SecurityException if a security manager exists and its <code>checkPropertyAccess</code>
   * method doesn't allow access to the specified system property.
   * @throws NullPointerException if <code>key</code> is <code>null</code>.
   * @throws IllegalArgumentException if <code>key</code> is empty.
   * @see #setProperty
   * @see java.lang.SecurityException
   * @see java.lang.SecurityManager#checkPropertyAccess(java.lang.String)
   * @see java.lang.System#getProperties()
   */
  public static String getProperty(String key) {
    checkKey(key);
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      sm.checkPropertyAccess(key);
    }

    return props.getProperty(key);
  }

  /**
   * Gets the system property indicated by the specified key.
   * <p>
   * First, if there is a security manager, its
   * <code>checkPropertyAccess</code> method is called with the
   * <code>key</code> as its argument.
   * <p>
   * If there is no current set of system properties, a set of system
   * properties is first created and initialized in the same manner as
   * for the <code>getProperties</code> method.
   *
   * @param key the name of the system property.
   * @param def a default value.
   * @return the string value of the system property, or the default value if there is no property
   * with that key.
   * @throws SecurityException if a security manager exists and its <code>checkPropertyAccess</code>
   * method doesn't allow access to the specified system property.
   * @throws NullPointerException if <code>key</code> is <code>null</code>.
   * @throws IllegalArgumentException if <code>key</code> is empty.
   * @see #setProperty
   * @see java.lang.SecurityManager#checkPropertyAccess(java.lang.String)
   * @see java.lang.System#getProperties()
   */
  public static String getProperty(String key, String def) {
    checkKey(key);
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      sm.checkPropertyAccess(key);
    }

    return props.getProperty(key, def);
  }

  /**
   * Sets the system property indicated by the specified key.
   * <p>
   * First, if a security manager exists, its
   * <code>SecurityManager.checkPermission</code> method
   * is called with a <code>PropertyPermission(key, "write")</code>
   * permission. This may result in a SecurityException being thrown.
   * If no exception is thrown, the specified property is set to the given
   * value.
   * <p>
   *
   * @param key the name of the system property.
   * @param value the value of the system property.
   * @return the previous value of the system property, or <code>null</code> if it did not have one.
   * @throws SecurityException if a security manager exists and its <code>checkPermission</code>
   * method doesn't allow setting of the specified property.
   * @throws NullPointerException if <code>key</code> or <code>value</code> is <code>null</code>.
   * @throws IllegalArgumentException if <code>key</code> is empty.
   * @see #getProperty
   * @see java.lang.System#getProperty(java.lang.String)
   * @see java.lang.System#getProperty(java.lang.String, java.lang.String)
   * @see java.util.PropertyPermission
   * @see SecurityManager#checkPermission
   * @since 1.2
   */
  public static String setProperty(String key, String value) {
    checkKey(key);
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      sm.checkPermission(new PropertyPermission(key,
          SecurityConstants.PROPERTY_WRITE_ACTION));
    }

    return (String) props.setProperty(key, value);
  }

  /**
   * Removes the system property indicated by the specified key.
   * <p>
   * First, if a security manager exists, its
   * <code>SecurityManager.checkPermission</code> method
   * is called with a <code>PropertyPermission(key, "write")</code>
   * permission. This may result in a SecurityException being thrown.
   * If no exception is thrown, the specified property is removed.
   * <p>
   *
   * @param key the name of the system property to be removed.
   * @return the previous string value of the system property, or <code>null</code> if there was no
   * property with that key.
   * @throws SecurityException if a security manager exists and its <code>checkPropertyAccess</code>
   * method doesn't allow access to the specified system property.
   * @throws NullPointerException if <code>key</code> is <code>null</code>.
   * @throws IllegalArgumentException if <code>key</code> is empty.
   * @see #getProperty
   * @see #setProperty
   * @see java.util.Properties
   * @see java.lang.SecurityException
   * @see java.lang.SecurityManager#checkPropertiesAccess()
   * @since 1.5
   */
  public static String clearProperty(String key) {
    checkKey(key);
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      sm.checkPermission(new PropertyPermission(key, "write"));
    }

    return (String) props.remove(key);
  }

  private static void checkKey(String key) {
    if (key == null) {
      throw new NullPointerException("key can't be null");
    }
    if (key.equals("")) {
      throw new IllegalArgumentException("key can't be empty");
    }
  }

  /**
   * Gets the value of the specified environment variable. An
   * environment variable is a system-dependent external named
   * value.
   *
   * <p>If a security manager exists, its
   * {@link SecurityManager#checkPermission checkPermission}
   * method is called with a
   * <code>{@link RuntimePermission}("getenv."+name)</code>
   * permission.  This may result in a {@link SecurityException}
   * being thrown.  If no exception is thrown the value of the
   * variable <code>name</code> is returned.
   *
   * <p><a name="EnvironmentVSSystemProperties"><i>System
   * properties</i> and <i>environment variables</i></a> are both
   * conceptually mappings between names and values.  Both
   * mechanisms can be used to pass user-defined information to a
   * Java process.  Environment variables have a more global effect,
   * because they are visible to all descendants of the process
   * which defines them, not just the immediate Java subprocess.
   * They can have subtly different semantics, such as case
   * insensitivity, on different operating systems.  For these
   * reasons, environment variables are more likely to have
   * unintended side effects.  It is best to use system properties
   * where possible.  Environment variables should be used when a
   * global effect is desired, or when an external system interface
   * requires an environment variable (such as <code>PATH</code>).
   *
   * <p>On UNIX systems the alphabetic case of <code>name</code> is
   * typically significant, while on Microsoft Windows systems it is
   * typically not.  For example, the expression
   * <code>System.getenv("FOO").equals(System.getenv("foo"))</code>
   * is likely to be true on Microsoft Windows.
   *
   * @param name the name of the environment variable
   * @return the string value of the variable, or <code>null</code> if the variable is not defined
   * in the system environment
   * @throws NullPointerException if <code>name</code> is <code>null</code>
   * @throws SecurityException if a security manager exists and its {@link
   * SecurityManager#checkPermission checkPermission} method doesn't allow access to the environment
   * variable <code>name</code>
   * @see #getenv()
   * @see ProcessBuilder#environment()
   */
  public static String getenv(String name) {
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      sm.checkPermission(new RuntimePermission("getenv." + name));
    }

    return ProcessEnvironment.getenv(name);
  }


  /**
   * Returns an unmodifiable string map view of the current system environment.
   * The environment is a system-dependent mapping from names to
   * values which is passed from parent to child processes.
   *
   * <p>If the system does not support environment variables, an
   * empty map is returned.
   *
   * <p>The returned map will never contain null keys or values.
   * Attempting to query the presence of a null key or value will
   * throw a {@link NullPointerException}.  Attempting to query
   * the presence of a key or value which is not of type
   * {@link String} will throw a {@link ClassCastException}.
   *
   * <p>The returned map and its collection views may not obey the
   * general contract of the {@link Object#equals} and
   * {@link Object#hashCode} methods.
   *
   * <p>The returned map is typically case-sensitive on all platforms.
   *
   * <p>If a security manager exists, its
   * {@link SecurityManager#checkPermission checkPermission}
   * method is called with a
   * <code>{@link RuntimePermission}("getenv.*")</code>
   * permission.  This may result in a {@link SecurityException} being
   * thrown.
   *
   * <p>When passing information to a Java subprocess,
   * <a href=#EnvironmentVSSystemProperties>system properties</a>
   * are generally preferred over environment variables.
   *
   * @return the environment as a map of variable names to values
   * @throws SecurityException if a security manager exists and its {@link
   * SecurityManager#checkPermission checkPermission} method doesn't allow access to the process
   * environment
   * @see #getenv(String)
   * @see ProcessBuilder#environment()
   * @since 1.5
   */
  public static java.util.Map<String, String> getenv() {
    SecurityManager sm = getSecurityManager();
    if (sm != null) {
      sm.checkPermission(new RuntimePermission("getenv.*"));
    }

    return ProcessEnvironment.getenv();
  }

  /**
   * Terminates the currently running Java Virtual Machine. The
   * argument serves as a status code; by convention, a nonzero status
   * code indicates abnormal termination.
   * <p>
   * This method calls the <code>exit</code> method in class
   * <code>Runtime</code>. This method never returns normally.
   * <p>
   * The call <code>System.exit(n)</code> is effectively equivalent to
   * the call:
   * <blockquote><pre>
   * Runtime.getRuntime().exit(n)
   * </pre></blockquote>
   *
   * @param status exit status.
   * @throws SecurityException if a security manager exists and its <code>checkExit</code> method
   * doesn't allow exit with the specified status.
   * @see java.lang.Runtime#exit(int)
   */
  public static void exit(int status) {
    Runtime.getRuntime().exit(status);
  }

  /**
   * Runs the garbage collector.
   * <p>
   * Calling the <code>gc</code> method suggests that the Java Virtual
   * Machine expend effort toward recycling unused objects in order to
   * make the memory they currently occupy available for quick reuse.
   * When control returns from the method call, the Java Virtual
   * Machine has made a best effort to reclaim space from all discarded
   * objects.
   * <p>
   * The call <code>System.gc()</code> is effectively equivalent to the
   * call:
   * <blockquote><pre>
   * Runtime.getRuntime().gc()
   * </pre></blockquote>
   *
   * @see java.lang.Runtime#gc()
   */
  public static void gc() {
    Runtime.getRuntime().gc();
  }

  /**
   * Runs the finalization methods of any objects pending finalization.
   * <p>
   * Calling this method suggests that the Java Virtual Machine expend
   * effort toward running the <code>finalize</code> methods of objects
   * that have been found to be discarded but whose <code>finalize</code>
   * methods have not yet been run. When control returns from the
   * method call, the Java Virtual Machine has made a best effort to
   * complete all outstanding finalizations.
   * <p>
   * The call <code>System.runFinalization()</code> is effectively
   * equivalent to the call:
   * <blockquote><pre>
   * Runtime.getRuntime().runFinalization()
   * </pre></blockquote>
   *
   * @see java.lang.Runtime#runFinalization()
   */
  public static void runFinalization() {
    Runtime.getRuntime().runFinalization();
  }

  /**
   * Enable or disable finalization on exit; doing so specifies that the
   * finalizers of all objects that have finalizers that have not yet been
   * automatically invoked are to be run before the Java runtime exits.
   * By default, finalization on exit is disabled.
   *
   * <p>If there is a security manager,
   * its <code>checkExit</code> method is first called
   * with 0 as its argument to ensure the exit is allowed.
   * This could result in a SecurityException.
   *
   * @param value indicating enabling or disabling of finalization
   * @throws SecurityException if a security manager exists and its <code>checkExit</code> method
   * doesn't allow the exit.
   * @see java.lang.Runtime#exit(int)
   * @see java.lang.Runtime#gc()
   * @see java.lang.SecurityManager#checkExit(int)
   * @since JDK1.1
   * @deprecated This method is inherently unsafe.  It may result in finalizers being called on live
   * objects while other threads are concurrently manipulating those objects, resulting in erratic
   * behavior or deadlock.
   */
  @Deprecated
  public static void runFinalizersOnExit(boolean value) {
    Runtime.runFinalizersOnExit(value);
  }

  /**
   * Loads the native library specified by the filename argument.  The filename
   * argument must be an absolute path name.
   *
   * If the filename argument, when stripped of any platform-specific library
   * prefix, path, and file extension, indicates a library whose name is,
   * for example, L, and a native library called L is statically linked
   * with the VM, then the JNI_OnLoad_L function exported by the library
   * is invoked rather than attempting to load a dynamic library.
   * A filename matching the argument does not have to exist in the
   * file system.
   * See the JNI Specification for more details.
   *
   * Otherwise, the filename argument is mapped to a native library image in
   * an implementation-dependent manner.
   *
   * <p>
   * The call <code>System.load(name)</code> is effectively equivalent
   * to the call:
   * <blockquote><pre>
   * Runtime.getRuntime().load(name)
   * </pre></blockquote>
   *
   * @param filename the file to load.
   * @throws SecurityException if a security manager exists and its <code>checkLink</code> method
   * doesn't allow loading of the specified dynamic library
   * @throws UnsatisfiedLinkError if either the filename is not an absolute path name, the native
   * library is not statically linked with the VM, or the library cannot be mapped to a native
   * library image by the host system.
   * @throws NullPointerException if <code>filename</code> is <code>null</code>
   * @see java.lang.Runtime#load(java.lang.String)
   * @see java.lang.SecurityManager#checkLink(java.lang.String)
   */
  @CallerSensitive
  public static void load(String filename) {
    Runtime.getRuntime().load0(Reflection.getCallerClass(), filename);
  }

  /**
   * Loads the native library specified by the <code>libname</code>
   * argument.  The <code>libname</code> argument must not contain any platform
   * specific prefix, file extension or path. If a native library
   * called <code>libname</code> is statically linked with the VM, then the
   * JNI_OnLoad_<code>libname</code> function exported by the library is invoked.
   * See the JNI Specification for more details.
   *
   * Otherwise, the libname argument is loaded from a system library
   * location and mapped to a native library image in an implementation-
   * dependent manner.
   * <p>
   * The call <code>System.loadLibrary(name)</code> is effectively
   * equivalent to the call
   * <blockquote><pre>
   * Runtime.getRuntime().loadLibrary(name)
   * </pre></blockquote>
   *
   * @param libname the name of the library.
   * @throws SecurityException if a security manager exists and its <code>checkLink</code> method
   * doesn't allow loading of the specified dynamic library
   * @throws UnsatisfiedLinkError if either the libname argument contains a file path, the native
   * library is not statically linked with the VM,  or the library cannot be mapped to a native
   * library image by the host system.
   * @throws NullPointerException if <code>libname</code> is <code>null</code>
   * @see java.lang.Runtime#loadLibrary(java.lang.String)
   * @see java.lang.SecurityManager#checkLink(java.lang.String)
   */
  @CallerSensitive
  public static void loadLibrary(String libname) {
    Runtime.getRuntime().loadLibrary0(Reflection.getCallerClass(), libname);
  }

  /**
   * Maps a library name into a platform-specific string representing
   * a native library.
   *
   * @param libname the name of the library.
   * @return a platform-dependent native library name.
   * @throws NullPointerException if <code>libname</code> is <code>null</code>
   * @see java.lang.System#loadLibrary(java.lang.String)
   * @see java.lang.ClassLoader#findLibrary(java.lang.String)
   * @since 1.2
   */
  public static native String mapLibraryName(String libname);

  /**
   * Create PrintStream for stdout/err based on encoding.
   */
  private static PrintStream newPrintStream(FileOutputStream fos, String enc) {
    if (enc != null) {
      try {
        return new PrintStream(new BufferedOutputStream(fos, 128), true, enc);
      } catch (UnsupportedEncodingException uee) {
      }
    }
    return new PrintStream(new BufferedOutputStream(fos, 128), true);
  }


  /**
   * Initialize the system class.  Called after thread initialization.
   */
  private static void initializeSystemClass() {

    // VM might invoke JNU_NewStringPlatform() to set those encoding
    // sensitive properties (user.home, user.name, boot.class.path, etc.)
    // during "props" initialization, in which it may need access, via
    // System.getProperty(), to the related system encoding property that
    // have been initialized (put into "props") at early stage of the
    // initialization. So make sure the "props" is available at the
    // very beginning of the initialization and all system properties to
    // be put into it directly.
    props = new Properties();
    initProperties(props);  // initialized by the VM

    // There are certain system configurations that may be controlled by
    // VM options such as the maximum amount of direct memory and
    // Integer cache size used to support the object identity semantics
    // of autoboxing.  Typically, the library will obtain these values
    // from the properties set by the VM.  If the properties are for
    // internal implementation use only, these properties should be
    // removed from the system properties.
    //
    // See java.lang.Integer.IntegerCache and the
    // sun.misc.VM.saveAndRemoveProperties method for example.
    //
    // Save a private copy of the system properties object that
    // can only be accessed by the internal implementation.  Remove
    // certain system properties that are not intended for public access.
    sun.misc.VM.saveAndRemoveProperties(props);

    lineSeparator = props.getProperty("line.separator");
    sun.misc.Version.init();

    FileInputStream fdIn = new FileInputStream(FileDescriptor.in);
    FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
    FileOutputStream fdErr = new FileOutputStream(FileDescriptor.err);
    setIn0(new BufferedInputStream(fdIn));
    setOut0(newPrintStream(fdOut, props.getProperty("sun.stdout.encoding")));
    setErr0(newPrintStream(fdErr, props.getProperty("sun.stderr.encoding")));

    // Load the zip library now in order to keep java.util.zip.ZipFile
    // from trying to use itself to load this library later.
    loadLibrary("zip");

    // Setup Java signal handlers for HUP, TERM, and INT (where available).
    Terminator.setup();

    // Initialize any miscellenous operating system settings that need to be
    // set for the class libraries. Currently this is no-op everywhere except
    // for Windows where the process-wide error mode is set before the java.io
    // classes are used.
    sun.misc.VM.initializeOSEnvironment();

    // The main thread is not added to its thread group in the same
    // way as other threads; we must do it ourselves here.
    Thread current = Thread.currentThread();
    current.getThreadGroup().add(current);

    // register shared secrets
    setJavaLangAccess();

    // Subsystems that are invoked during initialization can invoke
    // sun.misc.VM.isBooted() in order to avoid doing things that should
    // wait until the application class loader has been set up.
    // IMPORTANT: Ensure that this remains the last initialization action!
    sun.misc.VM.booted();
  }

  private static void setJavaLangAccess() {
    // Allow privileged classes outside of java.lang
    sun.misc.SharedSecrets.setJavaLangAccess(new sun.misc.JavaLangAccess() {
      public sun.reflect.ConstantPool getConstantPool(Class<?> klass) {
        return klass.getConstantPool();
      }

      public boolean casAnnotationType(Class<?> klass, AnnotationType oldType,
          AnnotationType newType) {
        return klass.casAnnotationType(oldType, newType);
      }

      public AnnotationType getAnnotationType(Class<?> klass) {
        return klass.getAnnotationType();
      }

      public Map<Class<? extends Annotation>, Annotation> getDeclaredAnnotationMap(Class<?> klass) {
        return klass.getDeclaredAnnotationMap();
      }

      public byte[] getRawClassAnnotations(Class<?> klass) {
        return klass.getRawAnnotations();
      }

      public byte[] getRawClassTypeAnnotations(Class<?> klass) {
        return klass.getRawTypeAnnotations();
      }

      public byte[] getRawExecutableTypeAnnotations(Executable executable) {
        return Class.getExecutableTypeAnnotationBytes(executable);
      }

      public <E extends Enum<E>>
      E[] getEnumConstantsShared(Class<E> klass) {
        return klass.getEnumConstantsShared();
      }

      public void blockedOn(Thread t, Interruptible b) {
        t.blockedOn(b);
      }

      public void registerShutdownHook(int slot, boolean registerShutdownInProgress,
          Runnable hook) {
        Shutdown.add(slot, registerShutdownInProgress, hook);
      }

      public int getStackTraceDepth(Throwable t) {
        return t.getStackTraceDepth();
      }

      public StackTraceElement getStackTraceElement(Throwable t, int i) {
        return t.getStackTraceElement(i);
      }

      public String newStringUnsafe(char[] chars) {
        return new String(chars, true);
      }

      public Thread newThreadWithAcc(Runnable target, AccessControlContext acc) {
        return new Thread(target, acc);
      }

      public void invokeFinalize(Object o) throws Throwable {
        o.finalize();
      }
    });
  }
}
